1. Field of the Invention
The present invention relates to a display device. The invention more particularly relates to a liquid crystal display device in which reduction in transmissivity is small even in a case where the screen is a high-definition screen, the liquid crystal display device having few pixel defects in the high-definition screen.
2. Description of the Related Art
In a liquid crystal display device, a thin film transistor (TFT) substrate in which pixels each including a pixel electrode, a TFT, and the like are formed like a matrix, a counter substrate facing the TFT substrate and having color filters formed at positions in facing relation to the pixel electrodes on the TFT substrate are arranged, and liquid crystal is sandwiched between the TFT substrate and the counter substrate. The liquid crystal display device forms images by controlling light transmissivity of liquid crystal molecules at each pixel.
The liquid crystal display device is flat and lightweight. Thus, the application of the liquid crystal display device has been extended in various fields. A small liquid crystal display device has widely been used in portable information terminals, such as mobile phones, smartphones, and Digital Still Cameras (DSCs). A viewing angle characteristic is a key issue in liquid crystal display devices. The viewing angle characteristic relates to a phenomenon that brightness or chromaticity changes between cases where a screen of the liquid crystal display device is viewed from front and where the screen is viewed from an oblique direction. An In-Plane Switching (IPS, which is a registered trademark of Japan Display Inc.) system causing a horizontal electric field to operate liquid crystal molecules has an excellent viewing angle characteristic.
Although various IPS systems exist, e.g., a system in which a common electrode is formed in a planar and solid manner and in which a comb-teeth pixel electrode is arranged above the common electrode so as to sandwich an insulating film between the pixel electrode and the common electrode, and to cause an electric field generated between the common electrode and the pixel electrode to rotate liquid crystal molecules. This system can increase the transmissivity. Accordingly, this system is currently a mainstream. The common electrode and an interlayer insulating film are formed on an organic passivation film serving also as a planarizing film.
Meanwhile, if a pixel size is reduced by providing the liquid crystal display device with a high-definition screen, a ratio of a radial cross-sectional area of a through-hole connecting between a pixel electrode and a source electrode of the TFT is increased.
In the above IPS liquid crystal display device, if the ratio of the radial cross-sectional area of the through-hole to the pixel size increases, an adhesion strength between the organic passivation film and the interlayer insulating film formed thereon decreases. Thus, a problem occurs, in which the interlayer insulating film peels off. JP-2011-59314-A describes a device which reduces stress on the interlayer insulating film and which prevents peeling-off of the interlayer insulating film by forming the interlayer insulating film not in the through-hole but only on the organic passivation film.
If a diameter of the through-hole is reduced as the pixel size is reduced by provision of the high-definition screen, a taper angle of a wall portion of the through-hole (hereinafter sometimes referred to also as a taper angle of the through-hole) should be increased. On the other hand, an alignment film is used to initially align the liquid crystal. A material of the alignment film, which is initially in a liquid state, is applied by flexographic printing, inkjet printing, or the like.
If the taper angle of the through-hole is increased, applying the material of the alignment film may cause a phenomenon that the material of the alignment film does not go into the through-hole. Then, a display defect, such as a luminance unevenness, occurs due to a fact that no alignment film exists in the through-hole, or to a thickness unevenness of the alignment film in a periphery of the through-hole. JP-2007-322563-A describes a device that varies a height at a periphery of the through-hole thereby to facilitate the alignment film to flow into the through-hole.
If the pixel size is reduced, a ratio of a pixel electrode to each pixel is relatively reduced, so that a transmissivity at each pixel is decreased. According to photoalignment of the alignment film initially aligning the liquid crystal, an alignment treatment can be performed on an inner wall portion of the through-hole. Thus, the inner wall portion of the through-hole can be used as a display area. JP-2013-140386-A describes a device that increases the transmissivity at each pixel by utilizing photoalignment to utilize an inside of the through-hole as a display area.